Ethanol can be produced from grain-based feedstocks (e.g. corn, sorghum/milo, barley, wheat, soybeans, etc.), from sugar (e.g. from sugar cane, sugar beets, etc.), and from biomass (e.g. from lignocellulosic feedstocks such as switchgrass, corn cobs and stover, wood or other plant material).
Biomass comprises plant matter that can be suitable for direct use as a fuel/energy source or as a feedstock for processing into another bioproduct (e.g., a biofuel such as cellulosic ethanol) produced at a biorefinery (such as an ethanol plant). Biomass may comprise, for example, corn cobs and stover (e.g., stalks and leaves) made available during or after harvesting of the corn kernels, fiber from the corn kernel, switchgrass, farm or agricultural residue, wood chips or other wood waste, and other plant matter (grown for processing into bioproducts or for other purposes). In order to be used or processed, biomass will be harvested and collected from the field and transported to the location where it is to be used or processed.
In a conventional ethanol plant producing ethanol from corn, ethanol is produced from starch. Corn kernels are cleaned and milled to prepare starch-containing material for processing. (Corn kernels can also be fractionated to separate the starch-containing material (e.g. endosperm) from other matter (such as fiber and germ). The starch-containing material is slurried with water and liquefied to facilitate saccharification where the starch is converted into sugar (e.g. glucose) and fermentation where the sugar is converted by an ethanologen (e.g. yeast) into ethanol. The product of fermentation (e.g. fermentation product) is beer, which comprises a liquid component containing ethanol and water and soluble components, and a solids component containing unfermented particulate matter (among other things). The fermentation product is sent to a distillation system. In the distillation system, the fermentation product is distilled and dehydrated into ethanol. The residual matter (e.g. whole stillage) comprises water, soluble components, oil and unfermented solids (e.g. the solids component of the beer with substantially all ethanol removed that can be dried into dried distillers grains (DDG) and sold as an animal feed product). Other co-products, for example syrup (and oil contained in the syrup), can also be recovered from the stillage. Water removed from the fermentation product in distillation can be treated for re-use at the plant.
In a biorefinery configured to produce ethanol from biomass, ethanol is produced from lignocellulosic material. Lignocellulosic biomass typically comprises cellulose, hemicellulose and lignin. Cellulose (a type of glucan) is a polysaccharide comprising hexose (C6) sugar monomers such as glucose linked in linear chains. Hemicellulose is a branched chain polysaccharide that may comprise several different pentose (C5) sugar monomers (such as xylose and arabinose) and small amounts of hexose (C6) sugar monomers (such as mannose, galactose, rhamnose and glucose) in branched chains.
The biomass is prepared so that sugars in the lignocellulosic material (such as glucose from the cellulose and xylose from the hemicellulose) can be made accessible and fermented into a fermentation product from which ethanol can be recovered. After fermentation, the fermentation product is sent to the distillation system, where the ethanol is recovered by distillation and dehydration. Other bioproducts such as lignin and organic acids may also be recovered as byproducts or co-products during the processing of biomass into ethanol. Determination of how to more efficiently prepare and treat the biomass for production into ethanol will depend upon the source and type or composition of the biomass. Biomass of different types or from different sources is likely to vary in properties and composition (e.g. relative amounts of cellulose, hemicellulose, lignin and other components). For example, the composition of wood chips will differ from the composition of corn cobs or switchgrass.
To completely realize the potential for converting biomass to ethanol, it is beneficial to more fully convert hemicellulose and cellulose from polysaccharides to sugars. Glucose and xylose are the primary sugars in most agricultural residues. It would be advantageous to provide for a system for biomass pretreatment that provides an efficient method of obtaining glucose from cellulose and xylose from hemicellulose in a form that preserves the potential for developing a crude xylose stream and a cellulose solid, which are both fermented.
Dilute acid pretreatment as a first step in a lignocellulosic process is an effective means of hydrolyzing a significant portion of the structural polysaccharides to monomer sugars and more easily digestible polysaccharide chains. A problem associated with dilute-acid pretreatment is poor fermentability of the resulting hydrolysates. A broad range of compounds are liberated and formed during the acid attack and many are toxic to the fermenting microorganism. Thus, it would be advantageous to provide for a new pretreatment process that minimizes the development of the inhibitors, reduces the energy requirements of the process, and mitigates the overall operating expenses of the corresponding ethanol production facility.